X-ray image diagnostic apparatus

ABSTRACT

Time for readout with no recording is shortened by performing readout with no recording only to the pixels having lags of the previous image. Also, a flat panel detector for receiving X-ray that has transmitted through an object to be examined, outputting an X-ray image, image memorizing means for memorizing said X-ray image, display means for displaying the X-ray image, a console for sending a start signal to read out the image and sending the threshold for extraction of the particular pixels to which readout with no recording is performed, readout control means for controlling readout of the image data of the flat panel detector  11 , previous image memorizing means for memorizing image data of the previous image of the flat panel detector and overwriting image data of the particular pixels to which readout with no recording has been performed, and readout address determining means for extracting the particular pixels according to said threshold within image data of previous image memorizing means and determining its address are comprised to obtain an image having no artifact. And readout control means controls readout of the flat panel detector so as to perform readout with no recording only to the particular pixels of which the address is determined.

FIELD OF INVENTION

[0001] The present invention relates to an X-ray image diagnosticapparatus for obtaining an X-ray image of a diagnosed part of an objectto be examined by detecting X-ray that has transmitted through theobject in a flat panel detector. More particularly, it relates to anX-ray image diagnostic apparatus for obtaining an image that has nomotion artifact by removing lags in the flat panel detector.

BACKGROUND OF THE ART

[0002] In an X-ray image diagnostic apparatus hitherto, an object to beexamined is irradiated with X-ray and the X-ray that has transmittedthrough the diagnosed part of the object is received with an X-raydetector, such as a flat panel detector (refer to Paul R. Granfors;“Performance Characteristics of an Amorphous Silicon Flat Panel X-rayImaging Detector.” Proc. SPIE Medical Imaging, February 1999. pp.480˜488), and then the X-ray image outputted from said X-ray detector isdisplayed on a CRT (cathode-ray tube) monitor or the like.

[0003] A flat panel detector used in an X-ray image diagnostic apparatusis comprised of a scintillator to transform X-ray that has transmittedthrough the object into light, a photo diode (an amorphous silicon type,for example) to transform the light outputted from said scintillatorinto electrical charge. This flat panel detector obtains an X-ray imageby reading out electrical charge of the photo diode through a switchingelement (TFT(Thin Film Transistor), for example).

[0004] In the flat panel detector mentioned above, it is known that evenafter reading out electrical charge from the photo diode, the chargethat is not read out remains in the photo diode. FIG. 5 is an exampleshowing how the electrical charge, or the lags, remaining in the photodiode decreases along with time. (For the detail, refer to P. L.Weisfield; “High Performance Amorphous Silicon Image Sensor for X-rayDiagnostic Medical Imaging Applications.” Proc. SPIE Medical Imaging,February 1999. pp. 307˜317.) As is clear in FIG. 5, the amount of theelectrical charge remaining in the photo diode hardly changes in a shorttime within a second for taking images of fluoroscopy and radiography.Then the remaining charge becomes lags that affect the next image and amotion artifact, which is an obstacle to diagnosis, appears on theimage.

[0005] For removing the lags of the previous image, the electricalcharge remaining in the photo diode of the flat panel detector is readout without memorizing it into an image memorizing means beforememorizing the next image from the flat panel detector into the imagememorizing means (hereafter, it is referred to as “readout with norecording.”)

[0006] However, in the conventional art mentioned above, for removingthe lags of the previous image, readout with no recording must beperformed to all photo diode of the flat panel detector, that is, to allpixels. Then said readout takes as much time as performing readout toone more image.

[0007] Also, when there exists a region with an extremely high signallevel in an X-ray image, the lags are not sufficiently removed byperforming readout with no recording only one time. Then, readout withno recording for several images has to be performed.

[0008]FIG. 6 is a figure for explaining an example of the above case andFIG. 6(a) is an example of a histogram of electrical charge stored inthe photo diode of the flat panel detector that has received the X-rayimage. In the FIG. 6(a), there exists a region with an extremely highsignal level in the center of the X-ray image. FIG. 6(b) shows thehistogram of electrical charge stored in the photo diode of the flatpanel detector after performing readout with no recording one time tothe X-ray image of FIG. 6(a). In said histogram, there still remains aregion with a high signal level. Then, for removing the lags of saidhigh signal level part, readout with no recording for several times hasto be performed.

[0009] The present invention is made concerning said circumstances. Thepurpose of said invention is to shorten the time for the readout with norecording by performing the readout with no recording only to the pixelshaving high level lags of the previous image, as well as to provide anX-ray image diagnostic apparatus to obtain an image of high quality byremoving motion artifact generated due to said lags.

DISCLOSURE OF THE INVENTION

[0010] To achieve said purposes, the X-ray image diagnostic apparatus ofthe present invention comprises:

[0011] an X-ray source for irradiating X-ray to the object to beexamined;

[0012] a flat panel X-ray detector for detecting X-ray that hastransmitted through said object and for generating X-ray image data,said flat panel X-ray detector being arranged opposite to said X-raysource;

[0013] image memorizing means for memorizing the X-ray image that isoutput above as image data;

[0014] display means for displaying the image data that is output aboveas an image;

[0015] operation means for outputting the start signal to read out theimage; and

[0016] readout control means for controlling the readout of the X-rayimage data from said flat panel X-ray detector when receiving the startsignal to read out the image,

[0017] wherein said operation means comprises means for determining itsreadout range by the histogram of remaining data after reading out X-rayimage data obtained at the previous time phase of the predetermined timephase from said flat panel X-ray detector to said image memorizingmeans. And said readout control means comprises means for controllingreadout of the remaining data in said flat panel X-ray detector from thereadout range that is determined above.

[0018] And in the X-ray image diagnostic apparatus comprising:

[0019] a console for outputting a start signal to read out the image;

[0020] a flat panel detector for receiving an image of X-ray that hastransmitted through the object to be examined and outputting said X-rayimage;

[0021] image memorizing means for memorizing an X-ray image outputtedfrom said flat panel detector as image data;

[0022] display means for displaying the image data memorized in saidimage memorizing means as an X-ray image; and

[0023] readout control means for controlling readout of the X-ray imagedata from said flat panel detector when receiving the start signal toread out the image outputted from said console,

[0024] said apparatus further comprises:

[0025] previous image memorizing means for memorizing the image data ofan X-ray image outputted from said flat panel detector, as well as forperforming readout with no recording to the particular pixels of saidflat panel detector before said flat panel detector outputs the nextimage data and overwriting said image data to which readout with norecording has been performed; and

[0026] readout address determining means for extracting the particularpixels from the image data memorized in said previous image memorizingmeans and for calculating the address corresponding to the position ofsaid particular pixels and outputting it to said readout control means.Said particular pixels are extracted according to the signal level ofthe image data, and said readout control means performs control in orderto perform readout with no recording only to the particular pixels ofsaid flat panel detector in accordance with the address of theparticular pixels inputted from said readout address determining means.

[0027] Compared with the conventional X-ray image diagnostic apparatus,the apparatus having said structure, to which previous image memorizingmeans and readout address determining means is added, can extract theparticular pixels to which readout with no recording is performedaccording to the signal level of the image data, as well as overwrite ofthe image data that is read out with no recording from the particularpixels of previous image memorizing means. Then, the condition ofremaining electrical charge at each pixel in the flat panel detectorbefore and after readout with no recording can be grasped. Besides, asthe address of the particular pixels to which readout with no recordingis performed can be determined in readout address determining means,readout with no recording only to the particular pixels can be performedthrough readout control means. Consequently, as readout with norecording is performed only to the particular pixels, time for readoutwith no recording can be shortened.

[0028] Also in the X-ray image diagnostic apparatus of the presentinvention, the particular pixels of said previous image memorizing meansis the pixels having a signal level equal to or above a particularsignal level (hereafter referred to as a threshold.) In said structure,among pixels in the flat panel detector the particular pixels to whichreadout with no recording is performed are extracted by using aparticular signal level as the threshold. Then the pixels having a highsignal level are read out with no recording. With said readout with norecording, readout of electrical charge begins from pixels having muchremaining charge in the flat panel detector. And so remaining charge isefficiently reduced.

[0029] Also in the X-ray image diagnostic apparatus of the presentinvention, extraction of the particular pixels of said previous imagememorizing means and readout with no recording of the particular pixelsof said flat panel detector are performed for a plural number of time.In said structure, extraction and readout with no recording of theparticular pixels is performed in the flat panel detector is executedmore than two times if electrical charge remaining in the flat paneldetector is not removed sufficiently by executing readout with norecording only one time. Consequently, remaining charge in the flatpanel detector is removed sufficiently and efficiently.

[0030] Also in the X-ray image diagnostic apparatus of the presentinvention, the threshold used as a standard for extracting theparticular pixels to which readout with no recording after the secondtime is performed from the image data of said previous image memorizingmeans is determined from the previous threshold. In said structure, asthe threshold for the standard of extracting the particular pixels towhich readout with no recording is performed can be determined inrelation to the previous threshold, the threshold can be determinedeasily by, for example, keeping the ratio between the two thresholds.Also, as the threshold is determined in relation to the signal level ofthe pixels of the flat panel detector, the condition of electricalcharge remaining in the flat panel detector can be grasped easily andthe remaining charge can be also reduced certainly.

[0031] Also in the X-ray image diagnostic apparatus of the presentinvention, the number of the particular pixels to which readout with norecording is performed is subtracted from the number of pixels for oneframe, and the resultant value of said subtraction is used as theparticular pixels to which readout with no recording after the secondtime is performed from the image data of said previous image memorizingmeans. In said structure, readout with no recording can be performed twotimes while reading out the image data for one frame. For example,readout with no recording can be performed efficiently by reading outpixels having a signal level equal to or above the threshold in thefirst readout with no recording and then reading out the image data indescending order of the signal level in the second readout with norecording.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032]FIG. 1 is a block diagram showing an embodiment of the X-ray imagediagnostic apparatus of the present invention,

[0033]FIG. 2 is a flow chart for explaining the process of finding outthe pixels having a signal level equal to or above the threshold amongthe image data memorized in previous image memorizing means 16 as theparticular pixels to which readout with no recording is performed,

[0034]FIG. 3 is a schematic diagram for explaining relation between thethreshold in readout with no recording and the image data memorized inprevious image memorizing means 16,

[0035]FIG. 4 is a flow chart for explaining the process of finding outthe address of the particular pixels to which the second readout with norecording is performed in readout address determining means 17,

[0036]FIG. 5 is an example showing how lags in the flat panel detectordecreases along with time,

[0037]FIG. 6 is a diagram showing change of lags in the flat paneldetector between before and after readout with no recording.

BEST MODE FOR CARRYING OUT THE INVENTION

[0038] Hereafter, an embodiment of the X-ray image diagnostic apparatusof the present invention will be explained using the attached drawings.

[0039]FIG. 1 is a block diagram showing one embodiment of an X-ray imagediagnostic apparatus of the present invention.

[0040] In FIG. 1, the X-ray image diagnostic apparatus of the presentinvention comprises:

[0041] a flat panel detector (a two-dimensional detector)11 forreceiving X-ray that has transmitted through an object to be examined(not described in the figure) and outputting an X-ray image;

[0042] image memorizing means 12 for memorizing an X-ray image outputtedfrom the flat panel detector 11 as image data;

[0043] display means 13 for displaying the image data memorized in theimage memorizing means 12 on a monitor;

[0044] a console 14 where an operator send readout control means 15 astart signal to read out the image of the flat panel detector 11;

[0045] readout control means 15 for controlling readout of the imagedata of the flat panel detector 11 when receiving the start signal toread out the image from the console 14;

[0046] previous image memorizing means 16 for memorizing the image datamemorized in image memorizing means 12, as well as overwriting the imagedata only of the particular pixels to which readout with no recordinghas been performed on said memorized image data after the image dataoutputted from the flat panel detector 11 by the control signal of saidreadout control means 15 is memorized in image memorizing means 12; and

[0047] readout address determining means 17 for performing readout withno recording only to the particular pixels of the flat panel detector 11by extracting only the particular pixels within the image data memorizedin the previous image memorizing means 16, and calculating the addresscorresponding to the position of those particular pixels, and thenoutputting the address of the particular pixels to readout control means15.

[0048] The characteristic of the present invention relates to the methodof performing readout with no recording to the image data in the flatpanel detector 11. And the elements concerning readout of the image datain the flat panel detector 11 in FIG. 1 are the flat panel detector 11,previous image memorizing means 16, readout address determining means 17and readout control means 15. Hereafter, the details of these elementswill be explained.

[0049] First, the flat panel detector 11 comprises a flat plate-shapedscintillator and a photo diode array, as mentioned in the background ofthe art. And electrical charge in the photo diode is read out through aswitching element. When an X-ray image is taken to the flat paneldetector 11, electrical charge corresponding to the X-ray dose histogramof the X-ray image is stored in each element of the photo diode array inthe flat panel detector 11. Therefore, each element of the photo diodearray corresponds to each pixel of the X-ray image and electrical chargestored in each element of the photo diode array corresponds to the pixellevel. Here, it substantially corresponds to the amount of electricalcharge that is read out from the photo diode through the switchingelement as image data because it takes time to read out electricalcharge stored in the photo diode completely.

[0050] Readout control means 15 performs the first readout control ofimage data of the flat panel detector 11 when receiving the start signalto read out the image from the console 14. The first image data that isread out is output to image memorizing means 12 and previous imagememorizing means 16. The image data outputted to image memorizing means12 is memorized at this point and is displayed on display means 13 to beused for medical diagnosis. The first image data outputted to previousimage memorizing means 16 is memorized at this point and is used forextraction of the particular pixels.

[0051] Besides, readout control means 16 performs the second readout,that is, readout with no recording of the image data only to theparticular pixels of the flat panel detector 11 according to the addressinformation of the particular pixels determined in readout addressdetermining means 17, and then outputs to previous image memorizingmeans 16. At this time, the second image data is not output to imagememorizing means 12. In previous image memorizing means 16, the secondimage data of the particular pixels is overwritten on the first imagedata.

[0052] Previous image memorizing means 16 is used for extracting theparticular pixels to which readout with no recording is performed aftermemorizing the first image data under the control of readout controlmeans 15. After the particular pixels are extracted, previous imagememorizing means 16 memorizes the second image data of these particularpixels. The image data of each time stored in previous image memorizingmeans 16 is used for extracting the particular pixels to which readoutwith no recording is performed next.

[0053] Readout address determining means 17 extracts the particularpixels according to the image data memorized in previous imagememorizing means 16 and determines readout address of the extractedparticular pixels to output to readout control means 15.

[0054] Extraction of the particular pixels is usually performedaccording to the image data, or the signal level, corresponding toelectrical charge stored in the photo diode of the flat panel detector11. As said signal level becomes larger, the lags also become larger.Therefore a predetermined signal level is set as a signal levelthreshold (hereafter referred to as a threshold) to extract pixelshaving a signal level equal to or above said threshold as the particularpixels.

[0055] Said threshold can be set directly at the console 14 as shown bythe actual line in FIG. 1. And it can be also set by separatelyproviding threshold setting means 18 as shown by the broken line inFIG. 1. When the latter method is used, the threshold can be set byproviding a threshold table in threshold setting means 18 and makingsaid threshold setting means 18 select a threshold within the table, orby providing expressions in threshold setting means 18 and making saidthreshold setting means 18 select a parameter. Also, the function ofsaid threshold setting can be provided in readout address determiningmeans 17 that extracts the particular pixels using the threshold.

[0056] Extraction of the particular pixels is performed possibly onetime or more than two times. At the first extraction, the threshold isusually set with a signal level. But after the second time, thethreshold is not necessarily set with a signal level. It might be setwith other items, for example, the number of pixels.

[0057] When a threshold is set with a signal level and this signal levelis high, the number of the particular pixels decreases, and so time forreadout is shortened. But, in this case, lags increase and readout withno recording has to be performed many times. And if the threshold is setwith a low signal level, the number of the particular pixels increases,and so time for readout is prolonged. Then readout with no recording isnot performed efficiently. Therefore, the method for setting thethreshold is important. In said embodiment, the value that the maxsignal level in the previous image data is multiplied with a constantrate is set as the threshold.

[0058] Next, the X-ray image diagnostic apparatus of the presentinvention in one embodiment shown in FIG. 1 will be explained. First, asmodes for taking X-ray images, the fluoroscopy mode and the radiographymode are used. In the fluoroscopy mode, an image of an object to beexamined is taken with low X-ray dose to determine the position forradiography mode and the X-ray image is read out continuously from theflat panel detector 11, and then the image is displayed as a dynamicimage on display means 13, such as a monitor. Compared with it, in theradiography mode, a clear image having a few noise elements is capturedwith a comparatively high X-ray dose at the position for imaging theobject determined in the fluoroscopy mode, and the X-ray image is readout from the flat panel detector 11. In said fluoroscopy mode, forexample, 30 frames of the X-ray image are read out per second from theflat panel detector 11 and are displayed on the display means 13.

[0059] After irradiating X-ray to the object, the operator inputs thestart signal to read out the X-ray image from the console 14 to readoutcontrol means 15. Then the X-ray image is output from the flat paneldetector 11 receiving X-ray that has transmits through the object and ismemorized in image memorizing means 12.

[0060] Now, the case where the radiography mode is changed to thefluoroscopy mode will be considered. In the radiography mode, the imagedata of the X-ray image outputted from the flat panel detector 11 ismemorized in image memorizing means 12. And then said image data is alsomemorized in previous image memorizing means 16. Here, as the signallevel of the image data (the image data of the X-ray image in theradiography mode) memorized in previous image memorizing means 16becomes higher, the lags that appear in the X-ray image displayed ondisplay means 13 become larger after changing to the fluoroscopy mode.

[0061] Then, readout address determining means 17 finds out pixelshaving a signal level equal to or above the threshold within image datamemorized in previous image memorizing means 16 and determines thesepixels as the particular pixels to which readout with no recording isperformed. The determined address is sent to readout control means 15.Here, the operator determines the threshold in advance consideringcharacteristics of the lags shown in FIG. 5 or the time for changingfrom the radiography mode to the fluoroscopy mode, and sends thethreshold from the console 14 (or threshold setting means 18) to readoutaddress determining means 17.

[0062] Readout control means 15 performs readout (readout with norecording) of the image data from the flat panel detector 11 accordingto the address of the particular pixels to which readout with norecording is performed that is sent from readout address determiningmeans 17, as well as overwrites the image data that is read out inprevious image memorizing means 16.

[0063] If the start signal to read out the X-ray image in thefluoroscopy mode is not promptly input from the console 14 afterfinishing readout with no recording, that is, if there is time untilchanging to the fluoroscopy mode in the next time phase, the secondreadout with no recording is performed using the image data that ismemorized in previous image memorizing means 16 and overwritten in thefirst readout with no recording. As the threshold for determining theaddress of the particular pixels to which the second readout with norecording is performed, a new threshold is set in the console 14 or inthreshold setting means 18 according to the threshold (a particularlevel) used in the first readout with no recording, and said newthreshold is sent to readout address determining means 17. The methodfor setting said new threshold will be explained later.

[0064] Said process of readout with no recording is repeated until thestart signal to read out the X-ray image in the fluoroscopy mode isinput from the console 14. By repeating the process, readout with norecording is performed only to the particular pixels for shortening thetime for it, as well as readout with no recording can be performed tothe broader range of image data when there is time until the other modeis input from the console 14.

[0065] Next, an example of operation of readout address determiningmeans 17 will be explained using FIG. 2. FIG. 2 is a flow chart forexplaining the process in readout address determining means 17 offinding out pixels having a signal level equal to or above the thresholdas the particular pixels to be read out with no recording within theimage data memorized in previous image memorizing means 16.

[0066] In FIG. 2, the image data memorized in previous image memorizingmeans 16 is read out one pixel at a time by the judging block 21 inreadout address determining means 17. In the judging block 21 the imagedata of the read pixels is compared with the predetermined threshold. Ifthe image data of the pixels that is read out from previous imagememorizing means 16 is larger than the threshold in process block 22, itis determined that said address of the pixels is used as the address ofthe particular pixels and is sent to readout control means 15. Readoutaddress determining means 17 performs said process repeatedly as manytimes as the number of pixels of the image data memorized in previousimage memorizing means 16.

[0067] Next, an example of a method of determining the threshold in thesecond readout with no recording will be explained.

[0068] The threshold T1 in the second readout with no recording is setas a new threshold by the console 14 or threshold setting means 18according to the threshold used in the first readout with no recordingand is sent to readout address determining means 17. Given that thethreshold in the first readout with no recording is T and the rate ofremaining electrical charge after reading out charge to the storedcharge in the photo diode of the flat panel detector 11 is W%, thethreshold T1 that is newly set in the second readout with no recordingis expressed with the expression below:

T1=T×(W/100)  (1)

[0069]FIG. 3 is a schematic diagram for explaining the relation betweenthe threshold in readout with no recording and the image data memorizedin previous image memorizing means 16. FIG. 3(a) shows a histogram ofany line of the image data memorized in previous image memorizing means16 before readout with no recording. In FIG. 3(a), the address of theparticular pixels corresponding to region α (region S2 and S4) equal toor above the predetermined threshold T is determined in readout addressdetermining means 17, and to each particular pixel readout with norecording is performed by readout control means 15 according to saidaddress.

[0070]FIG. 3(b) shows a histogram of any line of the image datamemorized in previous image memorizing means 16 after performing readoutwith no recording. Because the particular pixels that have been read outwith no recording is overwritten in previous image memorizing means 16,in the image data after readout with no recording memorized in previousimage memorizing means 16 shown in FIG. 3(b), the image data of theparticular pixels in regions S2 and S4 to which readout with norecording is performed have W % of the first signal level. And the imagedata of pixels in regions S1, S3 and S5 to which readout with norecording is not performed have the same signal level as the firstsignal level (equal to or below the threshold T).

[0071] Here, the console 13 or threshold setting means 18 sets a newthreshold T1(=T×(W/100)) according to the expression (1) and readoutaddress determining means 17 determines the address of the particularpixels corresponding to the region a (region S12, S2, S3, S4, S51 andS53) equal to or above said new threshold T1. And then readout controlmeans 15 performs the second readout with no recording according to saidaddress.

[0072] When readout with no recording is performed repeatedly, thethreshold after the third readout with no recording (T2, T3 . . . ) canbe set as T2=T1×(W/100) in the same way as the threshold T1(=T×(W/100))of the above second readout with no recording.

[0073] Next, another example of the method of determining the thresholdin the second readout with no recording in performing readout with norecording repeatedly will be explained. This example is to finish thefirst and the second readout with no recording within the same time ofreading out one image from the flat panel detector 11. According tothis, for example, given that the total number of the image data to beread out from the flat panel detector 11, that is, the number of pixelsfor an image is N and the number of the image data that is overwrittenon previous image memorizing means 16 in the first readout with norecording is n1, the number n2 of the image data to which the secondreadout with no recording is performed is expressed with the expressionbelow:

n2=N−n1  (2)

[0074] The threshold T1 in the second readout with no recording is usedas a signal level in the n2th pixel in arranging the image datamemorized in previous image memorizing means 16 at finishing the firstreadout with no recording in order of height of the signal level.Therefore, said threshold T1 is not determined automatically as in theexpression (1) and it has to be found out within the image datamemorized in previous image memorizing means 16.

[0075] Here, the process of determining the address of pixels to whichthe second readout with no recording is performed using the threshold insaid example in readout address determining means 17 will be explained.FIG. 4 is a flow chart for explaining the process of finding out theaddress of the particular pixels to which the second readout with norecording is performed in readout address determining means 17.

[0076] In FIG. 4, the image data that is overwritten with the image dataof readout with no recording after finishing the first readout with norecording is memorized in previous image memorizing means 16. At first,in the process block 41 the max signal level, Tmax is detected withinthe image data in previous image memorizing means 16. Next, in thejudging block 42 the image data that is read out from previous imagememorizing means 16 is compared with said max signal level Tmax. Whenthey have the same value, said image data is sent to the next judgingblock 43. If value of the image data that is read out from previousimage memorizing means is different from said max signal level Tmax, thevalue Tmax of the max signal level is modified in the process block 44.

[0077] Next, in the judging block 43 it is judged whether the sent imagedata is included in the number n2 of the image data to which the secondreadout with no recording is performed or not. When it is included inthe number n2 of image data to which readout with no recording isperformed, the address of previous image data is determined as theaddress of the particular pixels to which readout with no recording isperformed in the process block 45 and is sent to readout control means15. But when the sent image data is not included in the number n2 of theimage data to which readout with no recording is performed, (or the sentdata is over n2,) the process of finding out the address of theparticular pixels to which readout with no recording is performed isfinished in the process block 46. Said process is performed repeatedlyin the process block 47 until the number of pixels to which readout withno recording is performed reaches n2.

[0078] In said example, the case of performing readout with no recordingonly within the time of reading out one image of the flat panel detector11 has been explained. However, for example, if there is interval forseveral images between radiography and the next radiography and the timeof the interval is grasped as in the angiography, it is preferable torepeat said process by setting conditions so as to repeat readout withno recording for a certain number of times that is predetermined, or toset all the image data memorized in previous image memorizing means 16to be below the desired value Tmax. Here, the desired value Tmax isdetermined as a signal level that lags do not affect the next image.

[0079] Besides, although in said embodiment the case that the time phaseis changed from the radiography mode to the fluoroscopy mode has beenexplained, the present invention is of course applicable to in changingfrom the fluoroscopy mode to the radiography mode, in the interval untilthe next image is read out in reading more than two X-ray images withthe radiography mode and in the interval between the fluoroscopy modeand the radiography mode.

[0080] Also, although in said embodiment the case that image memorizingmeans and previous image memorizing means are set as separate memorizingmeans has been explained, it is of course possible to construct theapparatus so as to use a part of image memorizing means as previousimage memorizing means.

[0081] As explained above, according to the present invention it ispossible to select only the pixels having high level lags and to readoutwith no recording to them. Therefore, the X-ray image diagnosticapparatus that can shorten the time for readout with no recording can beprovided.

1. An X-ray image diagnostic apparatus comprising: an X-ray source forirradiating X-ray to an object to be examined; a flat panel X-raydetector for detecting X-ray that has transmitted through said objectand for generating X-ray image data, said flat panel X-ray detectorbeing arranged opposite to said X-ray source; image memorizing means formemorizing an X-ray image as image data; display means for displayingthe image data as an image; operation means for outputting a startsignal to read out the image; and readout control means for controllingreadout of the X-ray image data from the flat panel X-ray detector whenreceiving the start signal to read out the image outputted, wherein saidoperation means comprises means for determining its readout range bymeans of the histogram of remaining data after reading out X-ray imagedata obtained in the previous time phase of the predetermined time phasefrom said flat panel X-ray detector to said image memorizing means, andsaid readout control means comprises means for controlling readout ofthe remaining data in said flat panel X-ray detector based on thereadout range determined.
 2. An X-ray image diagnostic apparatus inclaim 1, wherein said readout control means reads out said particularpixels from said flat panel detector based on the address information ofthe particular pixels of X-ray image data remaining in said flat paneldetector in the previous time phase of the predetermined time phasedetermined in said means for determining said readout range.
 3. An X-rayimage diagnostic apparatus in claim 2, wherein extraction of saidparticular pixels is performed according to the signal level of X-rayimage data remaining in said flat panel X-ray detector.
 4. An X-rayimage diagnostic apparatus in claim 3, wherein extraction of theparticular pixels is performed by setting a predetermined signal levelas a signal level threshold (hereafter referred to as a threshold) andextracting pixels having a signal level equal to or above saidthreshold.
 5. An X-ray image diagnostic apparatus in claim 4, whereinthreshold setting means is provided and said threshold is set in saidthreshold setting means.
 6. An X-ray image diagnostic apparatus in claim5, wherein said threshold is set by providing a threshold table in saidthreshold setting means and making it select a threshold within thetable.
 7. An X-ray image diagnostic apparatus in claim 5, wherein saidthreshold is set by providing expressions in said threshold settingmeans and making it select a parameter.
 8. An X-ray image diagnosticapparatus comprising: an X-ray source for irradiating X-ray to an objectto be examined; a flat panel X-ray detector for detecting X-ray that hastransmitted through said object and generating X-ray image data, saiddetector is arranged opposite to said X-ray source; image memorizingmeans for memorizing an X-ray image outputted above as image data,display means for displaying image data memorized above as an image;operation means for outputting a start signal to read out the image; andreadout control means for controlling readout of X-ray image data fromsaid flat panel detector when receiving the start signal to read out theimage outputted above, characterized by further comprising: previousimage memorizing means for memorizing image data of an X-ray imageoutputted from said flat panel detector, as well as for performingreadout with no recording to the particular pixels of said flat paneldetector before said flat panel detector outputs the next image data andoverwriting said image data to which readout with no recording has beenperformed; and readout address determining means for extracting theparticular pixels from the image data memorized in said previous imagememorizing means and for calculating the address corresponding to theposition of said particular pixels to output it to said readout controlmeans, wherein said particular pixels are extracted regarding the signallevel of the image data as a standard, and said readout control meanscontrols so as to perform readout with no recording only to theparticular pixels of said flat panel detector according to the addressof the particular pixels inputted from said readout address determiningmeans.
 9. An X-ray image diagnostic apparatus in claim 2, wherein theparticular pixels of said previous image memorizing means are the pixelshaving a signal level equal to or above a particular value (hereafterreferred to as a threshold).
 10. An X-ray image diagnostic apparatus inclaim 9, wherein threshold setting means is provided to set saidthreshold in said threshold setting means.
 11. An X-ray image diagnosticapparatus in claim 10, wherein said threshold is set by providing athreshold table in said threshold setting means and selecting thresholdwithin the table.
 12. An X-ray image diagnostic apparatus in claim 10,wherein said threshold is set by providing expressions in said thresholdsetting means and selecting a parameters.
 13. An X-ray image diagnosticapparatus in claim 8, wherein extraction of the particular pixels ofsaid previous image memorizing means and readout with no recording tothe particular pixels of said flat panel detector are performed a pluralnumber of times.
 14. An X-ray image diagnostic apparatus in claim 9,wherein the threshold to be a standard for extracting the particularpixels to which readout with no recording after the second time isperformed from image data of said previous image memorizing means isdetermined from the previous threshold.
 15. An X-ray image diagnosticapparatus in claim 14, wherein the threshold to be the particular pixelsfrom said previous image memorizing means to which readout with norecording after the second time is performed is calculated bysubtracting the number of the particular pixels to which the firstreadout with no recording is performed from the number of pixels for oneframe of the particular pixels to which readout with no recording afterthe second time is performed.